The present invention relates to aluminum alloys prepared by extrusion, and more particularly to extruded Al-Si-Cu alloys and Al-Si-Cu-Mg alloys having a high silicon content and excellent in wear resistance and cuttability.
Throughout the specification and appended claims, the percentages used for the alloy components are all by weight.
Aluminum alloys having high strength, especially high wear resistance, are very useful for various mechanical parts which are subjected to great frictional forces, such as connecting rods of motor vehicle engines, power transmission pulleys, slippers, vanes and pistons of compressors, cylinder linings for engines, tape guides for tape recorders, synchronizer rings for speed change gears, etc., because the aluminum alloy is more lightweight than any other wear-resistant metal and therefore has various advantages.
A4032 alloy containing 11.0 to 13.5% of Si is already known as a wrought aluminum alloy having outstanding high-temperature characteristics. Although characterized by high resistance to heat and wear and a small coefficient of expansion, this wrought alloy is originally intended for forging and does not exhibit such characteristics before being forged. Thus the alloy material itself does not exhibit the above characteristics, while it is not noticeably excellent in cuttability. Accordingly the alloy has found greatly limited use only, for example, for pistons and cylinder heads.
Conventionally cast aluminum alloys are generally used for applications where especially high wear resistance is essentially required. Well known as such wear-resistant cast aluminum alloys are Al-Si alloys which contain about 10 to about 24% of Si and which include, for example, JIS-AC3A, -AC8A-C, -AC9A-B, etc. However, these alloys, which are cast, are limited in the shape of product, and it is difficult to obtain products of desired shape unlike wrought alloys. Accordingly they have the drawback of being limited in use. Moreover, because these alloy materials are prepared by casting, the primary Si crystals and eutectic Si crystals which are contained therein and serve as chief components for giving improved wear resistance are relatively coarse, have irregular shapes and are distributed unevenly. For example, the primary Si crystals are generally coarse and include those as large as about 150 microns in particle size, which the eutectic Si crystals are acicular and include those which are about 30 microns in length. These crystals are present as unevenly distributed. Because of these drawbacks, the cast alloys are not fully satisfactory in wear resistance or cutting properties. Although the particle size of primary Si crystals can be slightly reduced by an improvement treatment, the reduced sizes obtainable are limited to about 100 microns, while it is impossible to improve the eutectic Si crystals. Above all, it is impossible to correct the uneven distribution, so that the wear resistance of the alloy inevitably varies greatly from portion to portion.
In view of the above problems, research has been conducted extensively to obtain fine primary and eutectic Si crystals. As a result, Published Examined Japanese Patent Application No. 53-20242, for example, proposes to rapidly cool the molten alloy to be cast at a very high rate of 50.degree. C./sec to thereby inhibit the growth of crystals and give primary and eutectic Si crystals of greatly reduced sizes. It is reported that this prior-art method affords primary Si crystals of up to 40 microns in size if largest and eutectic Si crystals a majority of which are up to 20 microns in length. The specification of U.S. Pat. No. 4,077,810 also discloses a similar technique based on the same concept as above.
Nevertheless, my research has revealed that the greatest possible size reduction of Si particles, especially primary Si crystals, in the alloy structure does not always result in a proportional improvement in the wear resistance of the alloy. While the wear resistance of the alloy is provided by Si crystals which individually withstand the surface pressure resulting from friction, many experiments I have conducted show that the Si particles in the aluminum matrix, if excessively fine, rather exhibit reduced ability to withstand the frictional surface pressure, consequently failing to give improved wear resistance as contemplated.
Accordingly I have made investigations into particle size distributions of primary Si crystals and eutectic Si crystals which contribute to the greatest possible extent to the improvement of wear resistance and found such distributions to accomplish the present invention.